An abundant supply of fresh water is a fundamental requirement for municipal, industrial and agricultural uses. However, rising populations and source contamination have exerted increasing stress on fresh water supplies. Along with pressure from stricter regulations for clean water, improvements in water treatment technology are critical (Howe, K. J. and Tchobanoglous. G. (2005) Water Treatment: Principles and Design, John Wiley & Sons, Inc., Hoboken, N.J. 2nd edn.; Service, R. F. (2006) Science 313, 1088-1090).
Membrane-based filtration is the most important and widely used method for water purification due to its ability to completely and continuously filter impurities by size exclusion on a large scale (Howe, K. J. and Tchobanoglous. G. (2005) Water Treatment: Principles and Design, John Wiley & Sons. Inc., Hoboken, N.J., 2nd edn.). Generally, membranes can be categorized into four types: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) based on their pore size and ability to reject different matter.
UF membranes have an average pore size of ˜10 nm. In waste water treatment, UF membranes are used to reject pathogenic microorganisms such as viruses, bacteria, protozoa and other colloids (Cheryan, M. (1998) Ultrafiltration and Microfiltration Handbook, Technomic, Lancaster, Pa.). They can serve as a pretreatment step for desalination (Howe, K. J. and Tchobanoglous. G. (2005) Water Treatment: Principles and Design, John Wiley & Sons. Inc., Hoboken, N.J., 2nd edn.; Rosberg. R. (1997) Desalination 110, 107-114). UF membranes are commonly used for separations in the chemical, pharmaceutical, food and beverage industries, and are an integral component m blood dialysis.
When a membrane is used for separation, the flux gradually decreases during operation as the membrane is fouled by inorganic particulates, organic matter and/or biological microorganisms. The susceptibility to fouling varies among different membrane materials due to hydrophobic interactions between the foulant and the surface of the polymeric membrane. Thus, membrane hydrophilicity has been linked to a membrane's propensity to foul, i.e. hydrophilic membranes generally foul less than hydrophobic membranes (Mcverry, B. T. et al. (2013) Chem. Mater. 25, 3597-3602; Liao, Y., et al. (2014) Materials Horizons 1, 58-64). A more hydrophobic membrane allows foulants to adhere strongly via van der Waals interactions to the membrane surface that leads to irreversible membrane fouling (Hilal, N., et al. (2005) Separ. Sci. Technol. 179, 323-333).
Chemical cleaning is typically used to remove adhered organic matter and biofilms from the membrane surfaces. Cleaning treatments restore membrane performance to regain flux loss during operation due to membrane fouling. Common chemicals used for cleaning membranes include caustics, oxidants/disinfectants, acids, chelating agents and surfactants (Liu, C., et al. (2006) Membrane Chemical Cleaning: From Art to Science, Pall Corporation, Port Washington, N.Y. 11050, USA). Chlorine bleach (sodium hypochlorite), is popular in industry for its low cost, commercial availability and ability to effectively reduce fouling when added to the feed solution. Strong oxidants such as hypochlorite not only kill microorganisms, but also oxidize functional groups in natural organic matter into more water-soluble moieties, allowing the new species to be easily washed away during operation. However, strong oxidants simultaneously attach chemical bonds found within the polymeric membrane material, negatively affecting the membrane properties (Eykamp, W. (1995) Microfiltration and ultrafiltration. In Membrane Separation Technology: Principles and Applications, Elsevier Science: Amsterdam; Gitis, V., et al. (2006) J. Membr. Sci. 276, 185-192: Wienk, I. M., et al. (1995) J. Polym. Sci. Pol. Chem. 33, 49-54; Nystrom, M. and Zhu, H. (1997) J. Membr. Sci. 131, 195-205; Wolf, H. and Zydney, A. L. (2004) J. Membr. Sci. 243, 389-399; Zhu, H. and Nystrom, M. (1998) J. Membr. Sci. 138, 309-321).
The impact of chlorine cleaning on polyethersulfone (PES) membranes shows that chlorine can actually cause more severe fouling and increase the electronegativity of the membrane after cleaning (Arkhangelsky, E., et al. (2007) J. Membr. Sci. 305, 176-184; Rouaix, S., et al. (2006) J. Membr. Sci. 277, 137-147; Gaudichet-Maurin, E. and Thominette, F. (2006) J. Membr. Sci. 282, (98-204). This leads to chain scission of the polymer and deteriorates the mechanical strength of the membrane (Arkhangelsky, E., et al. (2007) J. Membr. Sci. 305. 176-184; Thominette, F., et al. (2006) Desalination 200, 7-8; Kuzmenko, D., et al. (2005) Desalination 179, 323-333). Chemical attack by chlorine on polyamide RO membranes results in membrane failure with enhanced passage of salt and water (Manohar, S. K. and Macdiarmid, A. G. (1989) Synthetic Met. 29, 349-356; Langer, J. J. (1990) Synthetic Met. 35, 295-300; Shin, J. S., et al. (2005) Synthetic Met. 151, 246-255; Shadi, L., et al. (2012) J. Appl. Polym. Sci. 124, 2118-2126). In response, attempts have been made to modify membrane materials in order to make them less susceptible to chlorine degradation. For instance, using a polyamide that contains a tertiary amide instead of a secondary amide results in a chlorine-resistant RO membrane (Scheme I) (Manohar, S. K. and Macdiarmid, A. G. (1989) Synthetic Metals 29, 349-356; Langer, J. J. (1990) Synthetic Metals 35, 295-300).
Conducting polymers and their derivatives have been extensively examined recently for their potential use in water treatment membranes due to their hydrophilic properties, thermal and chemical stability, low-cost, facile synthesis and ability to be modified by doping (McVerry, B. T., et al. (2013) Chem. Mater. 25, 3597-3602; Liao, Y., et al. (2014) Materials Horizons 1, 58-64; Liao, Y., et al. (2012) J. Colloid Interf. Sci. 386, 148-157; Bocchi, V., et al. (1991) J. Mater. Sci. 26, 3354-3355; Price, W. E., et al. (1999) Synthetic Materials 102, 1338-1341; Alargova, R. G., et al. (1998) Colloid Surface A 134, 331-342; Li, X., et al. (2008) J. Membr. Sci. 320, 143-150; Fan, Z., et al. (2008) J. Membr. Sci. 310, 402-408; Fan, Z., et al. (2008) J. Membr. Sci. 320, 363-371; Zhao, W., et al. (2011) J. Membr. Sci. 385-386, 251-262; Guillen, G. R., et al. J. Mater Chem. (2010) 20, 4621-4628). As one of the most widely studied conducting polymers, polyaniline (Pani) in its emeraldine oxidation state has been blended with the commercial UF membrane material polysulfone (PSf) to form composite UF membranes with enhanced hydrophilicity and permeability (Fan, Z. et al. (2008) J. Membr. Sci. 310, 402-408; Fan, Z., et al. (2008) J. Membr. Sci. 320, 363-371; Zhao, S., et al. (2011) J. Membr. Sci. 385-386, 251-262; Guillen, G. R., et al. J. Mater. Chem. (2010) 20, 4621-4628). Pure Pani can also form UF membranes that exhibit permeabilities 10 times higher than commercial PSf membranes, but no bovine serum albumin (BSA) rejection (Guillen, G. R., et al. J. Mater. Chem. (2010) 20, 4621-4628). By adding a secondary amine such as 4-methylpiperidine (4-MP) into the Pani casting solution, the rejection of Pam UF membrane increases, while the hydrophilicity decreases. Sulfonated Pani, a derivative of Pani, can be blended with PSf to form UF membranes which have shown excellent hydrophilicity and very high flux restoration after water washing (Mcverry, B. T., et al. (2013) Chem. Mater. 25, 3597-3602).
Several types of n-substituted Pani have been reported in the literature (Chevalier, J. W., et al. (1992) Macromolecules 25, 3325-3331; Yang, D. and Mattes, B. R. (2002) J. Polym. Sci Pol. Phys. 40, 2702-2713; Yang, D. and Mattes, B. R. (1999) Synthetic Met. 101, 746-749; Yang, D., et al. (2002) Macromolecules 35, 5304-5313; Manohar, S. K. and Macdiarmid, A. G. (1989) Synthetic Met. 29, 349-356). Among these, n-alkyl Pani is the most common form. However, alkyl groups are hydrophobic, so introducing them into Pani will result in some loss of hydrophilicity.
Filtration membranes are traditionally produced using polyvinylidene fluoride (PVDF). In order for these membranes to function properly, PVDF must be blended or co-polymerized with a hydrophilic moiety post membrane formation. While PVDF itself is extremely resilient to acid base, and chlorine, the hydrophilic group is not. Upon exposure to base or chlorine, as during membrane cleaning, the hydrophilic moiety is hydrolyzed and degraded. Over time, this causes the membrane to become more hydrophobic (fouling prone). In addition, the membrane loses miss and compacts thereby becoming less permeable. Despite the known disadvantages caused by blending the polymer with a hydrophilic group, materials for the preparation of filtration membranes that wholly circumvent these issues have yet to be reported. Herein, filtration membranes comprising material that is intrinsically hydrophilic and resistant to oxidative damage are described.